Physical Layer Data Encoding Transmission media Signals Bits to signal transformation  Timing (bit rate)  Synchronization

Data Encoding We represent information such as readable characters by coding them into sequences of zeros and ones There are a number of accepted code sets in use The most commonly used standards are ASCII, EBCDIC, and UNICODE

Physical Layer Transmission Media (chapter 4) Copper media Optical fiber Microwave Satellite Radio Signals and encoding Signals, timing (chapter 5) Digital Transmission (chapter 5) Analog Transmission (chapter6) Multiplexing (chapter 6)

Copper Media Twisted pairs (UTP)  Minimum 2 twisted, insulated copper wires  Multiple twisted pairs may be required for certain applications  Each set (1, 2, or more pairs) form a link  May be packaged in large bundles  Used for telephone  Digital data transmitted as digital signals or analog signals

Copper media  Telephones connect to central office with copper pairs  Primary intra-building wiring media  Cheapest in building distribution  Limited distance for data  Limited data rates  Data rates of up to 1 gigabit/sec possible depending on distance and quality of wire.  Work ongoing for 10 gigabits/sec standard

Twisted Pair Categories LevelApplication/bit rate Category 1 voice Category 2voice, data to 4 Mbps Category 310 Mbps Category 416 Mbps Category 5100 Mbps Cat 5E1000 Mbps with 4 pairs Category Mbps with 4 pairs Category 7 10 Gbps with 4 pairs Distance for data limited to 100 meters

Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP.

Twisted Pair Problems Attenuation Near End Cross Talk (NEXT)  Problem with unshielded twisted pairs  No shielding of pairs  Signals from one pair cause electrical interference with signals of another pair in the cable

EIA 568 Standard Standards for UTP cabling Different versions  EIA 568A  EIA 568B Termination in special connectors  RJ 11 (2 pairs)  RJ 12 (3 pairs)  RJ 45 (4 pairs)

EIA 568

EIA 568A

Pin # Ethernet 10BASE-T 100BASE- TX EIA/TIA 568A EIA/TIA 568B or AT&T 258A 1Transmit +White with green stripWhite with orange stripe 2Transmit -Green with white stripe or solid green Orange with white stripe or solid orange 3Receive +White with orange stripe White with green stripe 4N/ABlue with white stripe or solid blue 5N/AWhite with blue stripe 6Receive -Orange with white stripe or solid orange Green with white stripe or solid green 7N/AWhite with brown strip or solid brown White with brown stripe 8N/ABrown with white stripe or solid brown. Brown with white stripe or solid brown

EIA 568 Cabling Rules Avoid running near power cables Do not make sharp bends in cable Do not tie too tightly or crimp Keep cabling away from devices that introduce noise (fluorescent lights) Do not staple Never run outside buildings

Shielded Twisted Pairs (STP) Similar to UTP except a metal shield is wrapped around each pair Used in some networking Technologies  IBM Token Ring No longer in favor

STP

Coaxial Cable Two conductors  One inner conductor  Outer conduction shield Size of inner conductor may be as much as 50 time larger than conductor in a twisted pair cable Has been used for  Long distance telephone and television  Local Area Networks (LANs)  CATV cable television Can achieve very high data rates Less susceptible to interference and crosstalk Long distance amplifiers needed at various distances

Coaxial Cable

Ethernet Cable Based on a 1973 paper by Robert Metcalfe at MIT He developed Ethernet about 1975 while at Xerox Corporation Originally based on a specific coaxial cable Basically a ‘bus’ with multiple attachments Very successful IEEE develop a standard based on this called IEEE 802.3

Optical fiber Thin cable of glass which transmits light Not affected by electromagnetic interference Now used for most long distance communications  Voice, video, data Data rates of 10 Gbps now common 40 and 100 Gbps under development Distances of miles between repeaters possible Many fibers can be bundled into a single cable

Light Characteristics    angle of reflection  angle of refraction If  >  second medium has higher optical density If  <  first medium has higher optical density Index of refraction = Cos(  )/Cos(  )

Light Characteristics If IR < 1 Second medium less optically dense With a < some critical angle  NO refracted light  ALL light reflected Optical fiber  Core - very pure glass or plastic  Cladding - surrounds core - also glass or plastic - but optically less dense than core  Protective cover

Optical fiber

Optical fiber Multimode and Single Mode Multimode – multiple light beams at various angles Single light source Various connector standards (ST, SC, MT-RJ)

Wave Division Multiplexing (WDM) Each color (wave length) can carry same data rate as regular fiber Up to 40 Gbps

Microwave Terrestrial – ‘tower to tower’ – ‘mountain to mountain’ Line of site – miles apart Alternative to coax for long distance Used for telephone, television, data High data rates possible

Microwave

Satellite Space station used as relay Uses different frequencies for transmit receive Used for voice, television, data Broadcast medium Single transmission can be received by many stations

Geosynchronous Satellite Remains in a fixed position relative to earth Distance from earth to satellite? 22,236 miles or 35,785 Km Delay characteristics? How long does it take for a bit to go from one host to another? How long does it take send a message from host A to host B and get a response?

Satellite

Radio Broadcast medium Usually not point to point like microwave Range can be 25 – 30 miles An predecessor of Ethernet was based on radio University of Hawaii - ALOHA

Wireless LAN Operate on radio frequencies A number of standards now in effect (b) 2.4 Ghz band 11 Mbps (g)2.4 Ghz band 54 Mbps (a)5 Ghz band 54 Mbps (n) (draft)2.4 & 5 Ghz up to 300 Mbps Many other devices use these frequencies

Other physical layer media Broadband over Power Lines Infrared Lasers Low Earth Orbit Satellites